[WebLayer] Fix crash when opening popups

I was testing my in-progress popup change, and found a crash when
opening multiple popups.

Bug: 1090492
Change-Id: I2c3cb3cc655d13ce5476192117c4d863c9d4c044
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2225626
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Auto-Submit: Clark DuVall <cduvall@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774364}

weblayer/browser/tab_impl.cc

@@ -944,6 +944,7 @@ void TabImpl::OnUpdateBrowserControlsStateBecauseOfProcessSwitch(
   if (did_commit &&
       current_browser_controls_state_ ==
           content::BROWSER_CONTROLS_STATE_SHOWN &&
+      top_controls_container_view_ &&
       top_controls_container_view_->IsFullyVisible()) {
     // The top-control is fully visible, don't animate this else the controls
     // bounce around.